1. Field of the Invention
The present invention relates to a bidirectional optical amplifier that amplifies the counter-propagating signals simultaneously, and more particularly to a multi-stage bidirectional optical amplifier with shared mid-stage components and a means for suppressing the relative intensity noise caused by the multiple back reflections.
2. Description of the Related Art
The bidirectional signal transmission over a single fiber offers several advantages compared with a conventional the unidirectional signal transmission. It reduces the use of fiber infrastructure by a factor of two and enables to achieve a higher spectral efficiency since there are little nonlinear interactions between the counter-propagating signals in optical fibers.
However, the bidirectional transmission systems are susceptible to the optical back reflections. Especially, the relative intensity noise (RIN) induced by the multiple back reflections can degrade the system performance. The magnitude of the Relative intensity noise is proportional to the square of optical amplifier gain. Thus the Relative intensity noise limits the maximum available amplifier gains of the bidirectional transmission systems.
To overcome this problem, many efforts have been devoted to develop optical amplifiers with a means for suppressing relative intensity noise. Referring to appended drawings (FIG. 1˜FIG. 3), the prior optical amplifiers for bidirectional optical transmission systems are described hereinafter.
FIG. 1, FIG. 2 and FIG. 3 are the schematic diagrams of the prior bidirectional optical amplifiers with a means for suppressing relative intensity noise induced by the multiple back reflections.
A bidirectional optical amplifier described in FIG. 1 (U.S. Pat. No. 5,815,308) suppresses relative intensity noise by using the frequency tunable reflection attenuator (10) which is laid between two bidirectional optical amplifiers. The frequency tunable reflection attenuator (10) is implemented by using two band-pass filters (38a, 38b) with different pass bands, two optical isolators (18a, 18b) and two directional couplers (DC). Here, the two band-pass filters (38a, 38b) and the two optical isolators (18a, 18b) block the reflected light and thereby suppress the relative intensity noise.
In the figure, the numeric 12 represents a wavelength division multiplexer that couples the signal light and the pump light, 14 represents a pump light source, and 16 represents an Erbium-doped optical fiber.
A bidirectional optical amplifier described in FIG. 2 (C. H. Kim and Y. C. Chung, “2.5 Gb/sx 16-Channel bidirectional WDM transmission system using bidirectional Erbium-doped fiber amplifier based on spectrally interleaved synchronized Etalon filters,” IEEE Photon. Technol. Lett., vol. 11. no. 6, pp 745-747, June 1999) is implemented by combining two two-stage unidirectional optical amplifier by using two optical circulators (20a, 20b). Each of the unidirectional amplifiers includes a comb filter, synchronized Etalon filter (22a, 22b), at the mid-stage of it. The pass band and the stop band of a synchronized Etalon filters (22a) coincide with the stop band and the pass band of the other synchronized Etalon filters (22b). Here, the synchronized Etalon filters (22a, 22b) and the optical circulators (20a, 20b) suppress the relative intensity noise.
A bidirectional optical amplifier described in FIG. 3 (U.S. Pat. No. 6,018,404) is composed of four wavelength selective couplers (24a, 24b, 24c, 24d) and a unidirectional optical amplifier (26a). The signals entering into the amplifier are routed by the first and the second wavelength selective couplers (24a, 24b) to direct to the third the wavelength selective coupler (24c). The signals coupled by the wavelength selective coupler (24c) to propagate in same direction, and then amplified by an unidirectional optical amplifier (26a).
The output of the unidirectional optical amplifier (26a) is separated by the fourth wavelength selective coupler (24d) to direct to the first and the second wavelength selective couplers (24a, 24b) and then launched into transmission fibers connected at the both ends of the bidirectional amplifier.
Here, relative intensity noise is suppressed by the optical isolators installed in the unidirectional optical amplifier (26a) and the wavelength selective couplers (24a, 24b, 24c, 24d). To increase the signal bit rate, the number of multiplexed channels, or the transmission distance of the wavelength-division multiplexed bidirectional transmission systems, several supplementary means are required such as means for compensating chromatic dispersion of optical fiber, means for widening the gain-bandwidth of the optical amplifier, means for minimizing the accumulation of optical amplifier noise and so on.
In general, a dispersion compensating fiber is—usually used for compensating the chromatic dispersion, a gain flattening filter for widening the gain-bandwidth of optical amplifier, and an ASE rejection filter for preventing the accumulation of optical amplifier noise.
The components mentioned above are usually incorporated in the mid-stage of a multi-stage optical amplifier to minimize optical signal-to-noise ratio degradation. However, the bidirectional optical amplifiers described in FIG. 1˜FIG. 3 are not suitable to incorporate with these elements cost-effectively. For example, let's consider the case we employ a dispersion compensating fiber of which Rayleigh back scattering coefficient is much larger than that of a conventional single mode fiber.
In the amplifier described in FIG. 1, the dispersion compensating fiber must be used inside of the frequency tunable reflection attenuator (10) to prevent the reflected light by the dispersion compensating fiber. Thus we must use two dispersion compensating fibers to implement a bidirectional amplifier. The amplifier described in FIG. 2 employs two separate two-stage unidirectional optical amplifiers (26a), so that it also requires two separate mid-stage components including the dispersion compensating fibers.
In optical amplifier described in FIG. 3, the counter-propagating signals can share a dispersion compensating fiber if we use it at the mid-stage of the unidirectional optical amplifier (26a). In this case, however, the counter-propagating signals pass the dispersion compensating fiber in the same direction, which increases the nonlinear effects in the dispersion compensating fiber makes the nonlinear signals impairment more severe. Furthermore, each way signal passes through two wavelength selective couplers before and after amplification. The insertion loss of the wavelength selective couplers degrade the noise figure of the amplifier and decrease the maximum output power of the amplifier.